In modern communication systems, in many cases several separate devices are interconnected via a bus system in order to allow e.g. information exchange between the devices, controlling of devices, controlling of a system containing several devices, etc. A plurality of different bus systems has been proposed of which some have been optimized for specific areas of application, e.g. for automotive applications, process automation in large scale industrial applications, etc. Examples for known bus systems are high speed CAN, low speed CAN, LIN, FlexRay, etc.
In such bus applications, several requirements for bus signals exists of which some are mandatory and other may optionally be complied with in order to achieve enhanced performance. In many bus applications, such as in high speed CAN, two bus lines are used for data transmission in ISO/OSI layer 1. In CAN applications, these two bus lines are CANH (high voltage line) and CANL (low voltage line). Data signals are transmitted by using two different levels, a dominant level and a recessive level. For realizing these two levels, the two bus lines are driven symmetrically with respect to an intermediate voltage, as schematically depicted in FIG. 4. As can be seen in FIG. 4, when the bus signal on the bus lines is driven from the recessive level to the dominant level, the signals on both bus lines are changed with a specific slope.
In order to achieve low electromagnetic radiation emission, a very symmetrical output voltage is desired (high symmetry between the signal changes on the two bus lines). In case of a high symmetry, the emissions of the two bus lines will cancel each other and the net disturbance will be smaller as compared to the case of lower symmetry between the signals on the bus lines. In order to achieve high symmetry, the slopes of the two signal changes should correspond and the signal change should take place on both bus lines with the same delay.
WO 99/57810 describes a CAN transmitter for low slope. According to the teaching of this document, high symmetry is achieved by a specific arrangement of the drive circuit.
To achieve good immunity of the bus signal, a bus driver circuit should have a current source output. This is explained for a LIN transceiver in WO 02/073804 A2. In this document, use is made of a current source that has the desired waveform as a function of time in order to achieve an accurate current waveform.